Isothermal titration calorimetry (ITC) directly measures heat evolved in a biochemical reaction as a function of the molar reactant ratio. It can simultaneously determine all binding parameters within a single set of experiments, and thus provides an efficient, high-precision, label-free method for characterization of biomolecular reactions. ITC can be used for applications in fundamental sciences as well as drug discovery and biotherapeutics development.
Certain ITC instruments, however, have been limited by complicated structural design, slow thermal response, and large sample and reagent composition. These issues can potentially be addressed by miniaturization via Microelectromechanical Systems (MEMS) technology. MEMS-based calorimetric devices can have unique advantages over convention instruments, including reduced sample consumption, rapid time response, and improved throughput. While MEMS technology holds the potential in improved biocalorimetry for characterization of biomolecular interactions, this opportunity has not been fully explored. One issue with MEMS calorimetric devices can be the inadequate capability of handling liquid biochemical samples, representing by their general use for solid- or gas-phase samples or for liquid-phase samples without fluidic confinement or integrating with off-chip external flow cells.
By integrating microfluidic functionalities and MEMS-based thermal transduction, certain microfluidic calorimeters featuring sensitive detection of minimized volumes of liquid samples have been developed. The calorimetric sensing in such devices can be achieved through IR thermography, mechanical resonation, or integrated thin-film thermal sensors such as a resistor or a thermopile. In terms of microfluidic handling, there are generally two types of microfluidic calorimeters: flow-through calorimeters in which micro-chambers or channels are used as biological reactors while sample solutions are introduced by continuous flows, and droplet-based calorimeters in which discrete sample droplets are generated and transported to a surface for thermal detection. However, when used for characterization of biomolecular interactions, the flow-through calorimeters can consume considerable amount of samples and conduct measurements without well-defined volumes, making it difficult to obtain quantitative information associated with the reaction; while the droplet-based calorimeters can have complicated design due to on-chip droplet generation and manipulation, and can be affected by energy dissipation via evaporation.
In addition, with the function of titration, i.e., introduction of reactants at controlled molar ratios, incorporated into microfluidic calorimeters, there have been attempts of integrated ITC measurements of biochemical interactions on MEMS devices. By continuous in-channel delivery of reactants with varying molar rates to a flow-focusing junction, the heat flux upon the chemical reaction can be measured and used to calculate the enthalpy change. By varying the concentrations of the reactant solutions that were deposited to form individual droplets, the reaction heat can be measured as a function of reactants' molar ratio. Also by sequential injections of small droplets containing a reactant to a larger droplet containing the other reactant, the reaction heat per injection can be measured in real time. However, such devices can be generally difficult to accurately control the environment where the reactions are measured.